The signal frequency zones for information/communication appliances such as PHS's and cell phones and the clock frequencies for computer CPU's have reached the GHz zone in recent years. The trend toward the use of higher frequencies is significant.
In general, dielectric loss for an electrical signal in a wiring circuit is proportional to the product of the square root of the relative permittivity of the insulator used for the wiring circuit, the dielectric dissipation factor of the insulator, and the frequency of the signal to be used. Because of this, the higher the frequency of the signal is, the larger the dielectric loss is. Since dielectric dissipation factor attenuates electrical signals to impair reliability of the signals, it is necessary to use an insulating material having a low relative permittivity and a low dielectric dissipation factor.
Polyimide resins have generally been used extensively as insulating layers in wiring circuit boards, such as insulating base layers and insulating cover layers. In particular, polyetherimides are in frequent use as insulating materials for IC boards and printed wiring boards because they are excellent in heat resistance, electrical insulating properties, mechanical properties, etc. (see, for example, Patent Document 1 specified below).
However, most of the known polyetherimides are relatively high in relative permittivity and dielectric dissipation factor and, hence, there are limitations on the use of such polyetherimides as insulating materials for high-density multilayered integrated circuits or for circuits for high-speed high-frequency use because of the performances thereof. Techniques for compensating for such electrical properties of polyetherimides are hence known, which include a technique in which a poly(phenylene ether), which has a low relative permittivity and relatively excellent electrical properties although poor in mechanical strength, moldability and heat resistance, is blended with a polyetherimide (see, for example, Patent Document 2 specified below) and a technique in which an aromatic vinyl compound is blended as a crosslinking ingredient with a polyetherimide (see, for example, patent document 3 specified below). However, the polymers used in these polymer blends are mostly incompatible with the polyetherimides and, hence, the polymer blends obtained have not been sufficiently improved in mechanical properties and heat resistance.
Namely, no polyimide resin has been known which has all of a low relative permittivity, low dielectric dissipation factor and high heat resistance, and is hence suitable for use as an insulating material in wiring circuit boards for electronic appliances operated at high frequencies.
Patent Document 1: JP 2001-151935 A
Patent Document 2: JP 05-179140 A
Patent Document 3: JP 05-156159 A